It is a requirement for the use of semiconductors in such sensors that the conductivity should have a relatively low temperature dependence as well as a very strong change of conductivity (.DELTA..delta.) in response to the concentration of the gases just mentioned in exhaust gas. Engine exhaust gas contains as noxious components in the case of rich mixture mainly carbon monoxide (0.2 to 8% CO), whereas oxygen predominates in the case of lean mixtures (0.5 to 3% O.sub.2), while the NO.sub.x content for fuel-air ratios 1.01 &lt;.lambda.&lt;1.2 can at most reach 0.4%.
Since in the case of rich mixtures the detection of CO is of first importance among the requirements for producing a warning signal, in the case of lean mixtures the determination of the O.sub.2 concentration, for example for the control of a backfire trap or for operating an internal combustion motor on a mixture that is an lean as possible, requires responsiveness to free O.sub.2 as well as to CO, so that both these responses are of primary importance.
The conductivity change .DELTA..sigma. resulting from the equilibrium between gaseous oxygen and oxygen bound in the semiconducting oxide lattice is given according to semiconductor theory by the equation EQU .DELTA..sigma..about.p.sub.O.sbsb.2 .+-.1/n (1)
for the values n.gtoreq.4. For the case of conductivity change resulting from adsorption, for O.sub.2 the value of n can also be 2 (oxygen ions with a single negative charge), and/for CO the equation holds with n.gtoreq.2. This appears in curves 1 and 2 of FIG. 1, in which the measured conductivity (.sigma.) is plotted against the oxygen partial pressure (p.sub.O.sbsb.2) on a log-log graph (both scales logarithmic). Curve 1 shows the relationships in the case of a semiconductor of MgO with 15 mol % FeO at 900.degree. C. Curve 2, on the other hand, shows the corresponding relations for a semiconductor of MgO with 10 mol % CoO, likewise at 900.degree. C. In the case of curve 1, the relation found is: EQU d log .sigma./ d log p.sub.O.sbsb.2 =-1/5.75=-0.17
whereas this relation in the case of curve 2 is evaluated at +1/3.5=0.29. In the case of these pure semiconductor oxides there is accordingly found, for a change of the oxygen partial pressure of about 1/2 order of magnitude, a change of conductivity of only 1/3 order of magnitude. This relatively low change of the electrical conductivity is in general too small for warning signals within the range of concentration variations that occur in practice in exhaust gases and they can be utilized only with great electrical measurement expense and complication, provided that there is small temperature dependence of the electrical conductivity or provided that this temperature dependence is compensated by a second sensor that is electrically identical with the first. For a regulating or control operation, therefore, these cnductivity differences are insufficient.